The present invention relates to radio engineering, particularly to transmit-receive diversity methods and devices in code division multiple access (CDMA) communication systems.
One of the common problems to CDMA cellular radio communications systems is multipath fading that reduces system capacity. The techniques of fading mitigation applied today allow to use only a part of potential non-fading channel capacity. Hence, creation of new anti-fading techniques with the purpose of further CDMA system capacity improvement is very critical at the moment.
The most effective anti-fading technique is diversity reception using M space diversity receiving antennas. The space positions of the antennas are selected to ensure weak correlation of signal fadings in different antennas. A disadvantage to the receive space diversity is high price and increased dimensions of the equipment. This particularly hinders application of the space receive diversity in mobile terminals. Hence, it is presently an important problem to provide an alternative fading combat technique having the same level of efficiency as the receive space diversity systems.
Various transmit diversity methods are known presently where a signal is transmitted from two or more space diversity antennas as shown on FIG. 1. Two or more diversity antennas are usually installed at a base station to provide the receive diversity. In case of diversity transmission, these antennas are also used as transmitting ones. The known methods of diversity transmission allow to provide mitigation of the adverse fading effect when a signal is received at one antenna. These transmit diversity methods, however, are far less effective than the receive diversity ones.
It is known a transmit-receive method disclosed in U.S. Pat. No. 5,109,390, where a data stream is modulated on the transmitting side and then transmitted via one space diversity channel (via one antenna). On the receiving side, the input signal is demodulated recovering the initial data stream.
Since the data stream is transmitted via one space diversity channel only, the signal at the receiving point may disappear at some time intervals due to the fading in the channel. As a result, reception of such a signal is characterized by high error rate. Decreasing of the error rate requires some measures to be taken to increase signal-to-noise ratio (SNR) at the receiver input and as a result leads to degradation of the communication system capacity.
There is a method for delay transmit diversity (see xe2x80x9cSpace-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Constructionxe2x80x9d. Vahid Tarokh. IEEE Transactions on Information Theory, vol. 44, No.2, March 1998), in which the same wideband signal is transmitted without delay via a first antenna, and with delays relative to one another via the rest of antennas, the delay value being no less than one chip of the spreading pseudo noise (PN) sequence. In this case the signal on the receiving side resembles a multipath signal and can be processed by the conventional Rake-receiver. However, this method has a substantial drawback. The known PN spreading sequences have imperfect autocorrelation properties, so signals arriving at the receiving side with different time delays act as severe interference to one another.
A method for orthogonal transmit diversity (OTD) in a CDMA system and an algorithm for its application is disclosed in the UMTS-2000 standard project for cellular CDMA systems developed by ETSI-SMG2 xe2x80x9cSubmission of Proposed Radio Transmission Technologiesxe2x80x9d issued Jan. 29, 1998, pp. 51-52, section 5.6.3.1 xe2x80x9cOrthogonal Transmit Diversityxe2x80x9d.
The method according to the above solution is as follows.
A stream of user information symbols is split into two sub-streams, each sub-stream having different symbols of the initial stream. Each symbol in each sub-stream is repeated twice, thus doubling its duration. An orthogonal code with repetition period equal to the sub-stream symbol duration is assigned to each symbol sub-stream. Symbols of the first sub-stream are transmitted via a first antenna, and symbols of the second sub-stream are transmitted via a second antenna. Prior to signal transmission a conventional processing is performed which includes PN spreading and analog modulation. That way orthogonality is maintained between two and more output sub-streams. Note, that this OTD method does not require additional base station channelisation codes that represent one of the basic resources. The necessary orthogonal codes could be produced from one orthogonal code assigned to a user. Denote this code by Pk. Then two new orthogonal codes can be produced as [Pk, Pk], [Pk, xe2x88x92Pk]. The brackets denote the concatenation operation. The repetition period of the orthogonal codes thus generated is twice the duration of the code assigned to the user and is equal to the duration of a sub-stream binary symbol.
Based on the description of the algorithm, a device on the transmitting side, according to this method, may be accomplished as shown on FIG. 2. The device according to FIG. 2 comprises M signal transmit branches 1-1-1-M, two modulators 81-82 and two antennas 91-92. Each transmit branch contains binary symbol stream splitter 2, the input of which is the input of the device, orthogonal code generator 3, orthogonal modulator 4, the first inputs of which are connected to the corresponding outputs of binary symbol stream splitter 2, and the second inputsxe2x80x94to the outputs of orthogonal code generator 3, the outputs of orthogonal modulator 4 are linked to the first inputs of PN spreader 5, the second input of which is joined to the output of the spreading PN code generator, the outputs of PN spreader 5 are connected to the corresponding first inputs of first 81 and second 82 modulators, the second inputs of first 81 and second 82 modulators are connected to the outputs of pilot signal generator 7, the outputs of modulators 81 and 82 are joined to first 91 and second 92 antenna, respectively.
For signal receiving, the UMTS standard project proposes a device described in the UMTS-2000 standard project for cellular CDMA systems developed by ETSI-SMG2 xe2x80x9cSubmission of Proposed Radio Transmission Technologiesxe2x80x9d issued Jan. 29, 1998, pp. 51-52, section 5.6.3.1 xe2x80x9cOrthogonal Transmit Diversityxe2x80x9d. This device is accomplished as shown on FIG. 3.
The known device of FIG. 3 comprises multiplier 10, a first input of which is the information input of the device, and a second input is the second input of the device to which PN code is applied, the output of multiplier 10 is connected to the first inputs of multipliers 11, 13, 16 and 18, to the second inputs of which corresponding pilot codes of channels 1 to N are applied, the outputs of multipliers are joined to the inputs of corresponding combiners 12, 14, 17, and 19, the outputs of combiner 12 and combiner 14 are linked to the first and second inputs of multiplier 15, respectively, the outputs of combiner 17 and combiner 19 are connected to the first and second inputs of multiplier 20, respectively, the outputs of multipliers 15 and 20 are joined to the first and the second inputs of soft decision combining unit 21, respectively, the output of which is the output of the device.
The known device is similar to the Rake-receiver. Multipliers 10, 13 and combiner 14 represent correlator of the input signal to the pilot (reference) signal of the first antenna, multipliers 10, 18 and combiner 19 represent correlator of the input signal to the pilot signal of the second antenna.
The outputs of multipliers 15 and 20 are soft decisions on transmitted binary symbols in corresponding sub-streams.
Unit 21 combines soft decisions on symbols of two sub-streams into one stream of soft decisions.
The known device operates as follows.
On the transmitting side, an input binary symbol stream is applied to binary symbol stream splitter 2, where the stream is split into two sub-streams each containing different binary symbols of the initial stream (for example, b1 streams in the first sub-stream, b2xe2x80x94in the second sub-stream). In each sub-stream, each binary symbol is repeated twice, thus doubling the duration of each binary symbol as compared to the duration of initial stream binary symbol.
In unit 3, from user orthogonal code of the repetition period equal to the duration of initial stream binary symbol, two orthogonal codes of the repetition period twice the duration of initial stream binary symbol are generated. One of the generated orthogonal codes is assigned to one sub-stream and the other onexe2x80x94to the other sub-stream. These codes are supplied to the second input of orthogonal modulator 4, to the first input of which an output signal from binary symbol stream splitter 2 is sent.
In unit 4, the corresponding orthogonal codes are applied to the binary symbols of the sub-streams, respectively.
In unit 6, PN code is produced and applied to the second input of PN spreader 5, to the first input of which coded binary symbols of sub-streams are supplied.
In unit 5, coded binary symbols of each sub-stream are spread by the PN code, thus generating sub-streams of PN spread binary symbols.
Unit 5 passes the PN spread binary symbols of the first sub-stream to the first inputs of modulator 81, and the PN spread binary symbols of the second sub-streamxe2x80x94to the first inputs of modulator 82. The pilot signal of the corresponding space diversity channel generated in unit 7 is sent to the second inputs of modulators 81 and 82.
In units 81 and 82, PN spread binary symbols of the sub-streams and pilot signals are modulated and transmitted via the first 91 and second 92 antennas, respectively.
On the receiving side, a signal is received and analog demodulated.
Correlation of the input signal to the PN code and orthogonal codes of corresponding sub-streams is calculated using multipliers 10, 11, combiner 12, and multipliers 10, 16 and combiner 17.
Correlation of the input signal to pilot signals of space diversity channels is calculated using multipliers 10, 13 and combiner 14, which form correlator of the input signal to the pilot signal of the first space diversity channel, and multipliers 10, 18 and combiner 19, which form correlator of the input signal to the pilot signal of the second space diversity channel.
Multiplier 15 makes soft decision on transmitted binary symbol of the first sub-stream (b1).
Multiplier 20 produces soft decision on transmitted binary symbol of the second sub-stream (b2).
Unit 21 combines the two soft decision sub-streams into a single soft decision stream.
These known method and device are described for the case of two transmitting antennas. The cited document, however, indicates that the disclosed structure could be expanded for use with the number of antennas N=2n.
In these known method and device, mitigation of fading effect is achieved due to the combined use of convolutional encoding and transmit diversity. Prior to transmission, the binary symbol stream is divided into frames and, in each frame, information binary symbols are coded by convolutional code. On the receiving side, signals transmitted via different antennas experience independent fading. It means reducing the probability that signals from both the antennas during receiving will simultaneously disappear. Due to deep signal fading in the radio channel connecting one transmitting antenna with the receiving antenna, a part of frame symbols can be received erroneously. However, redundancy introduced during the coding allows to recover the information symbols based on the correctly received part of the frame. Hence, during the receiving, error rate of the prototype method and device [3] is less than that of the prior art method [1]. Since the signals transmitted via different antennas are orthogonal, they do not create interference to each other during processing at the receiver. Therefore, SNR per coded binary symbol in this method is higher than in the method disclosed in xe2x80x9cSpace-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Constructionxe2x80x9d by Vahid Tarokh published in IEEE Transactions on Information Theory, vol. 44, No.2, March 1998.
This known method, however, is much less efficient than the conventional receive space diversity, where each coded binary symbol of the frame is received via two or more space diversity channels (antennas). In the prototype method, each coded binary symbol is transmitted via one space diversity channel only, so, in the fading channel, reliability of receiving of each binary symbol is low. In order to improve reliability of receiving, it is necessary to increase the SNR that is equal to decreasing of capacity of the cellular communications system.
The object of the present invention is to provide a method of orthogonal receive-transmit diversity that improves CDMA communications system parameters, such as capacity and data transmission reliability through substantial mitigation of fading effects on the data receiving, and a reliable device implementing such a method.
This object can be achieved by a method of the orthogonal receive-transmit diversity, wherein, on the transmitting side, a spreading code is assigned to each information stream of binary symbols of each user, N transmit space diversity channels are formed, a pilot signal is generated for each transmit space diversity channel, on the receiving side, the transmitted signals are received and demodulated, N received vectors of pilot signals are determined, correlation of the input signal to the spreading code is calculated at the serial intervals of duration of a binary symbol of the received information stream, forming a sequence of correlation vectors, additionally, according to the present invention:
on the transmitting side, each information stream of binary symbols is split into serial information packets of N serial binary symbols,
serial-to-parallel transformation of binary symbols is performed in each serial information packet, thus forming parallel information packet comprising N symbols,
the parallel information packet is repeated N times, thus forming the serial-parallel information packet comprised of N parallel and N serial groups of binary symbols within the interval of the serial information packet duration,
for each serial group of the serial-parallel information packet of binary symbols, an orthogonal code comprising N binary symbols is generated, thus forming a serial-parallel packet of binary orthogonal code symbols that consists of N parallel and N serial groups of binary orthogonal code symbols,
binary symbols in the parallel groups of the serial-parallel information packet are reordered so that the binary symbols of each serial group are not repeated,
binary symbols in the serial-parallel binary orthogonal code symbol packet are reordered the same way as in the serial-parallel information binary symbol packet,
the serial-parallel information packet of the reordered binary symbols is scrambled by the serial-parallel packet of the reordered binary orthogonal code symbols, thus forming a serial-parallel packet of binary coded symbols,
a space diversity channel is assigned to each serial group of the coded binary symbols of the serial-parallel information packet, each coded binary symbol is spread by user spreading code, serial groups of spread coded binary symbols of each information stream of each user and corresponding pilot signals are modulated and simultaneously transmitted via N transmit space diversity channels,
on the receiving side: a sequence of correlation vectors is split into serial packets of N correlation vectors,
each correlation vector is multiplied by the complex conjugate of received pilot signal vectors, and real part of each product is collected, thus forming serial-parallel packet of correlation coefficients,
reordering, inverse to the reordering of serial-parallel information packet binary symbols on the transmitting side, is performed in the parallel groups of serial-parallel correlation coefficient packet,
a serial-parallel packet of binary orthogonal code symbols, corresponding to the serial-parallel packet of orthogonal code symbols on the transmitting side and containing N serial and N parallel groups of binary orthogonal code symbols, is generated,
correlation of the serial groups of serial-parallel correlation coefficient packet to the serial groups of serial-parallel orthogonal code symbol packet is calculated forming a parallel packet of N soft decisions corresponding to N binary symbols of parallel information packet on the transmitting side,
parallel-to-serial transformation of parallel packet of soft decisions is carried out producing an output soft decision stream.
Reordering of binary symbols in the parallel groups of serial-parallel information packet is reasonable to perform by dyadic shift so that the binary symbols of the K-th parallel group are dyadically shifted by Kxe2x88x921.
Scrambling of the serial-parallel information packet of reordered binary symbols by the serial-parallel packet of reordered binary orthogonal code symbols is preferably performed by summation by modulo two of each binary symbol of the serial-parallel information packet of reordered binary symbols with the corresponding binary symbol of the serial-parallel packet of reordered binary orthogonal code symbols.
The object of the present invention can be also achieved by that the orthogonal transmit-receive diversity device comprises, on the transmitting side, M parallel branches of the user signal transmission, pilot signal generator, N modulators, N antennas, where each signal transmit branch contains serially linked orthogonal modulator and PN spreader, the second input of the PN spreader is connected to the output of PN code generator, each of N outputs of the PN spreader is joined to the input of corresponding modulator, the output of each modulator is connected with the corresponding transmit antenna; on the receiving side, an antenna, the output of which is linked to the input of the demodulator, correlator, calculating the correlation of the input signal to user""s spreading code, N correlators, calculating the correlation of the input signal to pilot signals of space diversity channels, searcher, the first inputs of correlators and searcher are combined and linked to the output of demodulator, the second input of correlator, calculating the correlation of the input signal to user""s spreading code, is connected to the first output of reference signal generator producing user""s spreading code at the first output, the second inputs of N correlators are linked to the second outputs of reference signal generator producing channel pilot codes at the these outputs, the third output of reference signal generator is joined to the input of packet synchronizer, according to the present invention, on the transmitting side, the following is introduced into each signal transmit branch: a serial-parallel binary symbol packet generator, the input of which is the input of device, and the output is joined to the input of binary symbol reordering unit, serially linked serial-parallel binary orthogonal code symbol packet generator and binary orthogonal code symbol reordering unit, the second inputs of serial-parallel binary symbol packet generator, binary symbol reordering unit, serial-parallel binary orthogonal code symbol packet generator and binary orthogonal code symbol reordering unit are combined to form the second input of device, the outputs of binary symbol reordering unit and binary orthogonal code symbol reordering unit are connected to the first and second inputs of the orthogonal modulators, respectively.
On the receiving side there are introduced a serial correlation vector packet generator, the input of which is linked to the output of correlator, calculating the correlation of an input signal to user""s spreading code, and the outputxe2x80x94to the first input of serial-parallel correlation coefficient packet generator, the second inputs of serial-parallel correlation coefficient packet generator are joined with the outputs of N correlators, the second input of serial correlation vector packet generator and third input of serial-parallel correlation coefficient packet generator are combined and joined to the output of packet synchronizer, the output of serial-parallel correlation coefficient packet generator is connected to the input of correlation coefficient reordering unit, the output of correlation coefficient reordering unit is linked to the first input of correlator, the second input of correlator is connected to the output of serial-parallel binary orthogonal code symbol packet generator, the output of correlator is linked to parallel-to-serial transformation unit, the output of which is the output of device.
A unit of summation by modulo 2 is preferably used as the orthogonal modulator.
As a result of the newly proposed sequence of operations, on the transmitting side a combination of binary symbols of serial information packet emerges in each serial group. In other words, each serial group of binary symbols contains a set of all the symbols of the serial information packet but in different combinations. Reordering of binary symbols in serial-parallel binary orthogonal code symbol packet should be performed in the same manner as reordering of binary symbols in the serial-parallel information binary symbol packet. Upon completion of the operations of summation, spreading and modulation, serial groups of spread binary symbols, each containing the same set of binary symbols of input information stream, are simultaneously transmitted via different space diversity channels. Information streams transmitted via different space diversity channels do not create interference to each other, even though one and the same spreading code is used in each space diversity channel. As a result, even when due to fading a signal disappears in all the space diversity channels except one, information will not be lost because a complete information stream being transmitted via each space diversity channel. Combining of the information streams transmitted via the space diversity channels with independent fading thus significantly reduces error rate during information receiving without SNR increase at the receiver input. The obtained gain in noise stability can be used for communication system capacity increase.